This invention relates to bonding machines which bond input/output leads from integrated circuit chips to conductors on the top surface of a substrate; and more particularly, it relates to those bonding machines which compensate during the bonding operation for misalignments that are caused by differences in planarity between the substrates' top and bottom surfaces.
Integrated circuits are made on thin, flat silicon chips which range from about 1/8 inch to 1/2 inch on a side. Several of these chips are commonly attached by an epoxy to the top surface of a substrate on which electrical interconnections for the chips are patterned, and leads are bonded between bonding pads on the conductors and bonding pads on the chips. Some machines bond these leads as discrete wires one at a time, while other machines bond all of the leads from one hip simultaneously.
With the latter type bonding machine, the leads for each chip are preformed and held at a certain pitch by an insulator film such as a polyimide. These lead assemblies are commonly referred to as tape automated bonding assemblies or TAB assemblies. To attach the leads of a TAB assembly to the chip and the substrate, the bonding machine includes blades which are heated by an electric current and which squeezes the leads against the chip and the substrate at a certain pressure.
If, however, the pressure that is exerted on the leads by the bonding machine blades is too large, the leads will be squeezed towards each other and thereby cause a short circuit. Conversely, if the pressure that is exerted by the blades is too small, the resulting bond will be too weak which will result in an open circuit. Thus, it is very important to have the pressure that is exerted by the bonding blades be precisely controlled and distributed across the leads uniformly.
But such uniformity in the bonding pressure has been particularly difficult to achieve in the prior art. This is because the substrates on which the chips lie often have non-coplanar top and bottom surfaces; and in the prior art bonding machines, the substrates rested on their bottom surface while the bonding blade squeezed the leads against the chip and the substrate top surface. Consequently, the bonding blades and the substrate's top surface were at angles that varied from substrate to substrate depending upon the degree of non-coplanarity between the substrate's top and bottom surfaces.
In an effort to solve this problem, one type of prior art bonding machine by MCC of Austin, Texas, has its bonding blades mounted on a gimbal. In operation, the blades are moved towards the substrate's top surface until a portion of the blades makes initial contact with a portion of the leads on the top surface. Thereafter, further movement of the blades toward the substrate causes the blades to tilt on the gimbal until the plane of the blades coincides with the plane of the substrate's top surface.
A drawback, however, with such a gimbal-mounted bonding machine is that pressure must be applied to just a portion of the leads in order to make the plane of the blades and the plane of the substrate's top surface coincide. Consequently, it is inherent to the bonder's operation that pressure is not uniformly distributed across the leads at all times; and so with finely pitched leads, short circuits can result.
Another drawback with the gimbal-mounted bonder is that as the blades pivot on the gimbal in order to align themselves with the substrate's top surface, the pivotal motion moves the blades both perpendicular and parallel to the leads. And, due to the parallel component of this motion, the blades misalign themselves with respect to the bonding pads. Such misalignment can degrade the mechanical strength of a bond and thereby cause long-term reliability problems and open circuits.
Accordingly, a primary object of the invention is to provide an improved bonding machine in which all of the above prior art problems are eliminated.